6:4 For it is impossible [to restore and bring again to repentance] those who have been once for all enlightened, who have consciously tasted the heavenly gift and have become sharers of the Holy Spirit,

6:5 And have felt how good the Word of God is and the mighty powers of the age and world to come,

A. It's impossible to bring them back to repent again

"And then they abandoned their faith! It is impossible to bring them back to repent again,"

2:25 He must correct his opponents with courtesy and gentleness, in the hope that God may grant that they will repent and come to know the Truth [that they will perceive and recognize and become accurately acquainted with and acknowledge it],

Because, in the first place, there's no faith to be restored.

B. They are again crucifying the SON of GOD, and exposing Him to public shame

"because they are again crucifying the Son of God and exposing him to public shame."